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CARBOXYLIC ACIDS 1 12:13 PM

CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

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Page 1: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

CARBOXYLIC ACIDS

1 12:13 PM

Page 2: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Carboxylic Acids Introduction

2

Carboxylic acids are organic compounds containing the carboxyl

group (-COOH), wherein the hydroxyl group (-OH) is directly

attached to the carbonyl (C=O) group.

Carboxylic acids constitute one of the most frequently

encountered classes of organic compounds in nature. 12:13 PM

Page 3: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Natural Carboxylic Acids

3

A great many carboxylic acids are encountered in nature, mostly,

in fruits. Indeed carboxylic acids were among the first class of

organic compounds to ever be isolated from nature.

Edible carboxylic acids found in citrous fruits and fermented

milk generally have sharp flavours. 12:13 PM

Page 4: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Nomenclature of Carboxylic Acids

4

The common names of some basic carboxylic acids are derived

from Latin names that indicate the first original natural source of

the carboxylic acid.

Structure of Acid Natural Source Common

Name

Ants (Formica) Formic acid

Vinegar (Acetum) Acetic acid

Basic Fat (Propio) Propionic acid

Rancid butter (Butyrum) Butyric acid

Present in aValerian herb Valeric acid

Goat (Caper) Caproic acid

H C OH

O

CH3 C OH

O

CH3CH2 C OH

O

CH3CH2CH2 C OH

O

CH3CH2CH2CH2CH2 C OH

O12:13 PM

Page 5: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

IUPAC Nomenclature of Aliphatic Carboxylic Acids

5

IUPAC names of straight chain aliphatic carboxylic acids are

derived by adding the suffix –oic acid to the systematic name of

the parent hydrocarbon. They are named as alkanoic acids.

# of Carbons Structure & IUPAC

Name of Alkane

Structure & IUPAC

Name of Acid

1

Methane

Methanoic acid

2

Ethane

Ethanoic acid

3

Propane

Propanoic acid

H C H

H

H

H C OH

O

CH3 CH3 CH3 C OH

O

CH3CH2 CH3 CH3CH2 C OH

O

12:13 PM

Page 6: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Systematic Nomenclature of Substituted Carboxylic Acids

6

The systematic names of substituted aliphatic carboxylic acids

are derived by:

(i) First identifying the parent chain that contains most, if not

all, the carboxyl groups.

(ii) Number the parent chain from the carbon of the carboxyl

group i.e the carboxyl carbon is C-1.

(iii) Identify the substituents and assign each substituent a

locator/address number (2,3,4…etc.) consistent with the

numbering in the parent chain.

(iv) Arrange the names of the substituents in alphabetical order

in the systematic name of the poly-substituted carboxylic

acid. 12:13 PM

Page 7: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Systematic Nomenclature of Substituted Carboxylic Acids

7

Example

All other substituted aliphatic monocarboxylic acids are named

using the same sequence. 12:13 PM

Page 8: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

IUPAC Nomenclature of Substituted

Aromatic Carboxylic Acids

8

Substituted aromatic acids with one carboxyl group are named as

derivatives of benzoic acid, with the position of substituents

being cited using the locators (2,3 etc) according to their position

on the benzene ring relative to the carboxyl group. The carbon

on which the carboxyl group is attached is by convention C-1.

12:13 PM

Page 9: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Acidity of Carboxylic Acids

9

Carboxylic acids may to dissociate to a carboxylate anion and a

hydrogen ion (proton).

12:13 PM

Page 10: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Substituent Effects on the Acidity of Aliphatic Carboxylic Acids

10

Carboxylic acids dissociate to a carboxylate anion and a proton.

Any factors that stabilize the excess charge on the carboxylate

anion will enhance the acid dissociation constant and hence the

acidity of the carboxylic acid.

ALKYL GROUPS: EFFECTS OF ELECTRON-DONATING GROUPS

Name of Acid Structure pKa Effect

Methanoic acid HCO2H 3.8

Ethanoic acid CH3CO2H 4.7 Weakening acidity

Propanoic acid CH3CH2CO2H 4.9 Negligible effect

Heptanoic acid CH3(CH2)5CO2H 4.9 12:13 PM

Page 11: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Substituent Effects on the Acidity of Aliphatic Carboxylic Acids

11

ELECTRON WITHDRAWING GROUPS

Name of Acid Structure pKa Effect

Ethanoic acid CH3CO2H 4.7

Methoxyethanoic acid CH3OCH2CO2H 3.6 EWG’s increase

acidity Cyanoethanoic acid NC-CH2CO2H 2.5

Nitroethanoic acid O2N-CH2CO2H 1.7

The electron withdrawing groups disperse the negative charge of

the carboxylate anion thus stabilizing it and promoting its

formation. Note the enhanced acidity in substrates with electron-

withdrawing groups. 12:13 PM

Page 12: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Substituent Effects on the Acidity of Aliphatic Carboxylic Acids

12

The increase in acidity with increasing electronegativity of the

halogen is another manifestation of the effectiveness of charge

dispersal in the stabilization of the negative charge of the

carboxylate anion.

-HALOGENS: EFFECT OF ELECTRONEGATIVITY ON ACIDITY

Name of Acid Structure pKa Effect

Ethanoic acid CH3CO2H 4.7

Fluoroethanoic acid FCH2CO2H 2.6 -Halogen groups increase

acidity with increasing

electronegativity

Chloroethanoic acid ClCH2CO2H 2.9

Bromoethanoic acid BrCH2CO2H 2.9

12:13 PM

Page 13: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Substituent Effects on the Acidity of Aliphatic Carboxylic Acids

13

The substient effect is cummulative. The more the electron-

withdrawing groups the better the stabilization of the carboxylate

anion and the higher their acidities.

CUMMULATIVE EFFECT OF SUBSTITUENTS ON ACIDITY

Name of Acid Structure pKa Effect

Chloroethanic acid ClCH2CO2H 2.9 Substituent effects are

additive Dichloroethanoic acid Cl2CHCO2H 1.3

Trichloroethanoic acid Cl3CCO2H 0.9

12:13 PM

Page 14: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Synthesis of Carboxylic Acids Preparative Strategies

14

Highlighted below are some of the most common strategies by

which carboxylic acids are prepared.

The common starting materials for the preparation of carboxylic

acids are primary alcohols, aldehydes and alkenes.

12:13 PM

Page 15: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Synthesis of Carboxylic Acids Oxidation of Primary Alcohols to Carboxylic Acids

15

The synthesis of carboxylic acids requires the generation or

incorporation of the carboxyl group in a substrate.

R CH2OHOxidizing Agent

R CO2H

Although, primary alcohols can be oxidized to carboxylic acids

using strong oxidizing agents such as CrO3, Na2Cr2O7 , K2Cr2O7

or KMnO4, in practice, however, the oxidation of primary

alcohols with acidic chromic acid solutions usually forms esters

(acid-catalysed esterification between the carboxylic acid and the

unreacted primary alcohol takes place) making this strategy

synthetically inefficient.

12:13 PM

Page 16: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Synthesis of Carboxylic Acids Oxidation of Primary Alcohols to Carboxylic Acids

16

The best conditions for the oxidation of primary alcohols to

carboxylic acids is under the basic conditions employing

potassium permanganate.

Example

12:13 PM

Page 17: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Synthesis of Carboxylic Acids Oxidation of Aldehydes to Carboxylic Acids

17

Aldehydes can be oxidised to carboxylic acids by a variety of

oxidizing agents. Both strong and mild oxidizing agents may be

employed successfully.

R CHOOxidizing Agent

R CO2H

Note that Ag2O does not oxidize alcohols 12:13 PM

Page 18: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Synthesis of Carboxylic Acids Oxidation of Aldehydes to Carboxylic Acids

18

Some aldehydes may contain other functional groups that are

sensitive to oxidation. The selective oxidation of such aldehydes

requires the use of mild and selective oxidizing agents for

aldehydes such as silver(I)oxide.

Oxidations of aldehydes with KMnO4 in basic media yields a

carboxylate salt that must be acidified to provide the free

carboxylic acid.

12:13 PM

Page 19: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Synthesis of Carboxylic Acids Ozonolysis of Alkenes to Carboxylic Acids

19

Ozonolysis of appropriately substituted alkenes under oxidative

cleavage (H2O2) conditions provides carboxylic acids.

12:13 PM

Page 20: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Reactions of Carboxylic Acids

20

Most of the reactions of carboxylic acids can be placed into three

categories:

(1) Reactions at the acidic hydrogen on the carboxyl group.

(2) Reactions at the carbonyl group

(3) Reactions at the carboxylate oxygen

The reactions of carboxylic acids can be directed to various sites

on the carboxyl group.

12:13 PM

Page 21: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Reaction of Carboxylic Acids with Sodium Bicarbonate

21

Example

Most carboxylic acids (pKa 5) are stronger acids than carbonic

acid (H2CO3) (pKa 6.4). Consequently they displace carbonic

acid from its salts (hydrogen carbonates). The most reliable test

for carboxylic acids employs NaHCO3 leading to evolution of

CO2. This is commonly called the bicarbonate test for carboxylic

acids.

C

O

OH + C

O

O +NaHCO3 Na H2O

Benzoic acid Sodium benzoate

+ CO2

12:13 PM

Page 22: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Reactions of Carboxylic Acids with Strong Bases

22

Example

Bases such as metal hydroxides (NaOH and KOH) and amines

abstract the acidic proton on carboxylic acids to form

carboxylate salts.

Mechanism

R C

O

O +H R C

O

O +NaOH Na H2O

R C

O

O + OHH R C

O

O + H O H

C

O

OH + C

O

O +NaOH Na H2O

Benzoic acidSodium benzoate12:13 PM

Page 23: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Acid-Catalysed Esterification of Carboxylic Acids

23

The acid-catalyst can be provided by strong mineral acids such

as H2SO4, HCl and H3PO4 or organic acids such as

benzenesulphonic acid or p-toluenesulphonic acid.

The traditional method for converting carboxylic acids to esters

is through an acid-catalyzed esterification in the presence of an

alcohol: Commonly referred to as the Fischer esterification.

R C

O

OH + R'OH R C

O

OR' + H2OH2SO4

Example

12:13 PM

Page 24: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Mechanism of the Acid-Catalysed Esterification of Carboxylic Acids

24

Step 2: Nucleophilic attack of the alcohol to the activated

carbonyl and proton tranfer

Step 1: Protonation of the carbonyl oxygen of the carboxylic

acid (activation of the carbonyl carbon).

Step 3: Loss of water to give the conjugate acid of the ester and

regeneration of acid catalyst.

R C

O

OH H+ R C

O

OH

H

AcidCatalyst

Protonatedcarboxylic acid

12:13 PM

Page 25: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Reaction of Carboxylic Acids with Amines

25

Amines, being organic bases, react with carboxylic acids to form

ammonium salts.

R C

O

O +H R C

O

ON HR'

R'N HR'

R'

H

Ammonium salt

R C

O

O +H R C

O

ON HR'

R'N HR'

R'

H

Ammonium salt

Mechanism

Example

C

O

OH + C

O

O

Benzoic acid

N

Et

H

Et

N

Et

H

Et

H

12:13 PM

Page 26: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Reaction of Carboxylic Acids with Amines in Presence of DCC

26

Primary and secondary amines react with carboxylic acids in the

presence of DCC to form amides.

DCC serves to activate the carboxyl group of the carboxylic acid

to aid in coupling to the amino group.

+C OHR

O

1,3-Dicyclohexylcarbodiimide

+ C

OHN

HN

Dicyclohexylurea(DCHU)

R' NH2 CHNR

O

+ CN N

(DCC)

R'

Amide

N,N-Diethyl-m-toluamide (Deet) is a common mosquito & tick

repellent readily made from m-toluic acid using DCC.

Page 27: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Mechanism of Coupling Carboxylic Acids with Amines Using DCC

27

The DCC serves to activate the hydroxyl group attached to the

carbonyl of the carboxyl group; thus converting it to a good

leaving group.

CN N + C OR

O

H CN N

H

+ C OR

O

CN N

H

+C OR

OCN

N

O

H

C R

O

C N

N

O

H

CR

O

R' N +R'

NH

R

O

+ C

OHN N

C

O

NHN+

+ C

OHN

HN

H

R'N

H

R

O

R'NR

O

HH

HH

Page 28: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Reaction of Carboxylic Acids with Phosphorus Pentachloride

28

Carboxylic acids react with phosphorus pentachloride to provide

acid chlorides.

Example

The success of the reaction depends on the strength of the P=O

bond that is formed in phosphorus oxytrichloride.

12:13 PM

Page 29: CARBOXYLIC ACIDS...Carboxylic Acids Introduction 2 Carboxylic acids are organic compounds containing the carboxyl group (-COOH), wherein the hydroxyl group (-OH) is directly attached

Reduction of Carboxylic Acids to Primary Alcohols

29

Carboxylic acids are reduced to primary alcohols when treated

with a strong reducing agent such as LiAlH4.

Medium strength reducing agents such as sodium borohydride

(NaBH4) that reduce aldehydes and ketones are not sufficiently

strong to reduce carboxylic acids.

R CO2HLiAlH4

R CH2OH(1)

(2) H2O or Dilute acid

Ph CO2H Ph OHLiAllH4

(1)

(2) H2O

Example